KR20170037646A - Enhanced activation of self-passivating metals - Google Patents

Abstract

A workpiece made of a self-passivating metal having at least one surface area that forms a Beilby layer as a result of a previous metal forming process may contain a workpiece in the vapor produced by heating the anoxic nitrogen halide salt And is activated for subsequent cold gas curing by exposure.

Description

[0001] ENHANCED ACTIVATION OF SELF-PASSIVATING METALS [0002]

Cross reference of related application

This application claims the benefit of U.S. Provisional Patent Application No. 62 / 031,338, filed July 31, 2014, the entire disclosure of which is incorporated herein by reference.

Conventional Carburizing  process

Conventional (high temperature) carburizing is an industrial process that is widely used to improve the surface hardness of molded metal articles ("case hardening"). In a typical commercial process, the workpiece is contacted with a carbon-containing gas at a high temperature (e.g., above 1,000 degrees Celsius), whereby carbon atoms liberated by decomposition of the gas diffuse into the workpiece surface. Curing is a process in which the diffused carbon atoms react with one or more metals in the workpiece to form separate chemical compounds, i. E., Carbides, which are then precipitated as distinct extremely hard crystalline particles in the metal matrix forming the workpiece surface . See Stickels' "Gas Carburizing" (pp 312-324, Volume 4, ASM Handbook, 1991, ASM International).

Stainless steel has corrosion resistance because it forms an impermeable layer of chromium oxide immediately upon exposure to the atmosphere. The river is said to be self-passivating.

When stainless steels are typically carburized, the chrome content of the steel is locally depleted through the formation of carbide precipitates responsible for surface hardening. As a result, chromium in the vicinity of the surface immediately surrounding the chromium carbide precipitate is insufficient to form protective chromium oxide on the surface. The corrosion resistance of the steel is impaired, so that the stainless steel is hardly surface hardened by the conventional (high ds) carburizing treatment.

Low temperature Carburizing  process

In the mid-1980s, a technique for surface hardening stainless steels was developed, in which the workpiece was brought into contact with a low temperature carbon-containing gas, typically below 550 ° C. At such a temperature, if the carburizing process is not continued for too long, carbon atoms liberated by the decomposition of the gas are diffused into the surface of the workpiece usually to a depth of 20 to 50 탆 without forming carbide precipitates. Nevertheless, a surface (surface layer) having high hardness is obtained. Since carbide precipitates are not produced, the corrosion resistance of the steel is not damaged and even improved. Such techniques, referred to as "low temperature carburization ", are described in many documents including US 5,556,483, US 5,593,510, US 5,792,282, US 6,165,597, EPO 0787817, JP 9-14019 (open 9-268364) and JP 9-71853 (open 9-71853) ≪ / RTI >

nitrification  And Carbo-nitriding  process

In addition to the carburizing treatment, nitriding and carbo-nitriding treatment can be used for surface hardening of various metals. The nitriding treatment essentially consists of a carburizing treatment and a nitriding treatment, except that instead of using a carbon-containing gas which decomposes to produce carbon atoms for the expression hardening, a nitrogen-containing gas decomposes to produce nitrogen atoms for surface hardening It works in the same way.

However, as with the carburizing treatment, if the nitriding treatment is achieved without rapid quenching at high temperatures, the curing takes place through the formation and precipitation of a separate compound of the diffusing atoms, i. On the other hand, if the nitriding treatment is achieved at low temperatures without plasma, the hardening takes place without the formation of precipitates through the stress applied on the crystal lattice of the metal by the nitrogen atoms diffused into the crystal lattice of the metal. As in the case of carburizing treatment, stainless steel loses intrinsic corrosion resistance of the steel when chromium in the steel reacts with diffusing nitrogen atoms to form a nitride, and therefore, it is usually subjected to nitriding treatment by a conventional (high temperature) or plasma nitriding treatment It does not.

In the carbo-nitriding process, the workpiece is exposed to both nitrogen and carbon-containing gases, whereby both nitrogen and carbon atoms diffuse into the workpiece for surface hardening. Like the carburizing and nitriding treatment, the carbo-nitriding treatment can be achieved at a high temperature at which curing takes place through the formation of nitride and carbide precipitates, or by interstitially dissolved nitrogen and carbon atoms diffused into the crystal lattice of the metal, Lt; RTI ID = 0.0 > curing < / RTI > through a rapidly localized stress field generated in the crystal lattice of the substrate. For convenience, all three of these processes, namely carburizing, nitriding and carbo-nitriding, are collectively referred to as "low temperature surface hardening" or "low temperature surface hardening process" in this disclosure.

Activation

Because the temperature involved in low temperature surface hardening is too low, carbon and / or nitrogen atoms will not penetrate the chrome oxide protective coating of the stainless steel. Thus, the low temperature surface hardening of such metals usually precedes the activation ("depassivation") step prior to that step, in which the workpiece is exposed to HF, HCl, NF ₃ , Containing gas such as F ₂ or Cl ₂ such that the oxide protective coating of the steel is permeable to the passage of carbon and / or nitrogen atoms.

WO 2006/136166 (US 8,784,576) to Somers et al., The disclosure of which is incorporated herein by reference, discloses a modified process for the low temperature carburization of stainless steels wherein acetylene is used as an activating component in the carburizing gas, That is, as a source compound for supplying carbon atoms for the carburization process. As shown in that document, a separate activation step using a halogen-containing gas is unnecessary since the acetylene source compound has sufficient reactivity to de-passivate the steel. Therefore, the carburizing technique of the document can be regarded as self-activating.

Christiansen et al., WO 2011/009463 (US 8,845,823), the disclosure of which is incorporated by reference herein, discloses a similar modified process for carburizing nitrides of stainless steels, in which case "N / C < / RTI > compound "is used as the source compound to supply the nitrogen and carbon atoms necessary for the carbo-nitridation process. The technique of the document can also be regarded as self-activation, since a separate activation step using a halogen-containing gas is unnecessary.

Surface preparation and Veil layer ( Beilby  Layer)

Low temperature surface hardening is often done for workpieces with complex shapes. In order to produce such a shape, several types of metal forming processes (e.g., sawing, scraping, machining) and / or wrought processing steps (e.g. forging, drawing, bending, etc.) Is normally required. As a result of such steps, contaminants, such as lubricating oil, moisture, oxygen, as well as structural defects in the crystalline structure, are often introduced into the area near the surface of the metal. As a result, in most workpieces having complex shapes, a defective surface layer with a fine grain structure and a considerable level of contamination than necessary caused by plastic deformation is usually produced. The layer, which can reach a thickness of 2.5 [mu] m and is known as a Beilby layer, is formed directly below a coherent chromium oxide protective layer or other passivating layer of stainless steel and other self-passivating metals.

As mentioned above, the traditional method of activating stainless steel for low temperature surface hardening is by contact with halogen containing gas. This activation technique is inherently unaffected by the veil overlay.

However, this can not be said for the self-activation technique described by Somers et al. And Christiansen et al. In which the workpiece is activated by contact with acetylene or "N / C compound ". Rather, unless a complex shaped stainless steel workpiece is surface treated by electrolytic polishing, mechanical polishing, chemical etching, etc., to remove the bailed layer before the surface hardening begins, the self-activating surface hardening technique of the literature is not effective at all, Even if somewhat effective, at best, we have experienced irregular and inconsistent results between surface areas. Thus, as a practical matter, the self-activating surface hardening techniques can not be used in a complex shaped stainless steel workpiece unless the workpiece is first pretreated to remove the bale overlay.

The problem with the present invention is that, when applied to a self-passivating metal with a bailed layer originating from a previous metal forming process in a conventional known self-activating low temperature surface hardening process, the problem is not only to activate the workpiece, Can be overcome by selecting an anoxic nitrogen halide salt as the source compound that supplies the nitrogen atoms and optionally the carbon atoms needed for the reaction.

Particularly, according to the present invention, the low-temperature nitridation and the low-temperature carbo-nitriding treatment are carried out by nitriding or nitriding if the source compound used for supplying nitrogen atoms for nitriding (together with carbon atoms for nitriding) It has been found that the workpiece to be carbo-nitrided can be self-activated even if it is made of a self-passivating metal having a bail non-layer originating from the previous metal forming process.

Thus, in one embodiment, the present invention provides, in one embodiment, a workpiece made of a self-passivating metal having at least one surface area that includes a bailed layer as a result of a previous metal forming process for a subsequent cold carburization, carburizing or nitriding treatment The process comprising exposing the workpiece to contact with a vapor produced by heating the oxygen-free nitrogen halide salt to a temperature high enough to convert the oxygen-free nitrogen halide salt to steam, The workpiece is exposed to the vapor for a time sufficient to activate the workpiece at an activation temperature that is lower than the temperature at which it forms nitride and / or carbide precipitates.

In addition, in another embodiment, the present invention provides a process for simultaneously activating and nitriding a workpiece made of a self-passivating metal having at least one surface area that forms a bailed layer as a result of a previous metal forming process, The process comprises exposing the workpiece to contact with the vapor produced by heating the oxygen-free nitrogen halide salt to a temperature high enough to convert the oxygen-free nitrogen halide salt to steam, the workpiece having nitrogen atoms entering into the surface of the workpiece Is exposed to the vapor at a nitridation temperature that is sufficiently high to cause diffusion but is lower than the temperature at which nitride precipitates are formed, thereby nitriding the workpiece without the formation of nitride precipitates.

In another embodiment, the present invention provides a process for simultaneously activating and carbo-nitriding a workpiece made of a self-passivating metal having at least one surface area that forms a bailed layer as a result of a previous metal forming process, The process comprises exposing the workpiece to contact with a vapor produced by heating the carbon containing oxygen-free nitrogen halide salt to a temperature high enough to convert the carbon-containing oxygen-free nitrogen halide salt to steam, the workpiece comprising nitrogen and Is exposed to the vapor at a carburizing nitridation temperature that is sufficiently high to allow carbon atoms to diffuse into the surface of the workpiece but is lower than the temperature at which nitride precipitates or carbide precipitates are formed, thereby nitriding the workpieces without forming nitride or carbide precipitates.

Definitions and Terms

As noted above, the fundamental difference between conventional (hot) surface hardening and the new low temperature surface hardening first developed in the mid 1980's is that in conventional (hot) surface hardening, carbide and / or nitride precipitates As shown in FIG. On the other hand, in low temperature surface hardening, hardening is the result of stresses exerted on the crystal lattice of the metal at the surface of the metal as a result of the carbon and / or nitrogen atoms diffused into the surface of the metal. Carbide and / or nitride precipitates responsible for surface hardening in conventional (hot) surface hardening treatments are not found in surface hardened stainless steels by low temperature carburizing treatment, and the low temperature surface hardening does not adversely affect the corrosion resistance of stainless steels As a result, it was originally thought that in the low-temperature carburizing process, the surface hardening occurs only as a result of the abrupt local stress field generated by the interstitial dissolved carbon and / or nitrogen atoms diffused into the steel (austenite) crystal structure.

However, in more recent, higher-level analytical studies, it has been found that, if low-temperature surface hardening is performed on an alloy of which some or all of its volume is composed of a ferrite phase, some types of conventionally unknown nitride and / or carbide precipitates are present in the ferrite phase As shown in FIG. Specifically, recent analytical work suggests that in the AISI 400 series of stainless steels, which generally represent ferrite phase textures, a small amount of nitride and / or carbide may be precipitated, which was previously unknown when the alloy was cold cured at low temperatures. Likewise, recent analytical work suggests that a small amount of nitride and / or carbide, previously unknown, may precipitate in the ferrite phase of the steel at low temperature surface hardening in duplex stainless steels containing both ferrite and austenite phases . It is known that the precise nature of the newly discovered nitride and / or carbide precipitates, previously unknown, is not known yet, but the chromium content is not depleted in the ferrite matrix immediately surrounding such "para-equilibrium & . As a result, the corrosion resistance of the stainless steel remains unimpaired because the chromium responsible for corrosion resistance remains uniformly distributed throughout the metal.

Thus, for the purposes of this disclosure, the term " workpiece surface layer without essentially nitride and / or carbide precipitates ", "surface hardened workpiece" without the formation of nitride and / or carbide precipitates, or temperature at which nitrides and / or carbide precipitates are formed Quot; lower temperature ", it is assumed that this is responsible for the surface hardening in the traditional (hot) surface hardening process, while the chromium content sufficient to lose corrosion resistance as a result of depletion of the chromium content in the metal matrix immediately surrounding the precipitate Quot; refers to a nitride and / or carbide precipitate of a type having a content of < RTI ID = 0.0 > Such references do not refer to previously discovered newly discovered nitride and / or carbide precipitates that may be formed in small quantities on ferrites of AISA 400 stainless steels, duplex stainless steels, and similar alloys of Gata.

alloy

The present invention can be practiced on any metal or metal alloy that is self-passivating in the sense of forming a coherent, high chromium oxide protective layer that is not permeable to the passage of nitrogen and carbon atoms upon exposure to air. Such metal alloys are known and described in, for example, prior patent documents relating to low temperature surface hardening processes, examples of which are described in US 5,792,282, US 6,093,303, US 6,547,888, EPO 0787817 and Japanese Patent Publication 9-14019 268364).

Particularly important alloys are stainless steel, i.e., 5 to 50 wt% nickel, preferably 10 to 40 wt% nickel, and chromium sufficient to form a protective layer of chromium oxide on the surface upon exposure to air, typically about 10% Or more of chromium. Preferred stainless steels contain 10 to 40 wt% Ni and 10 to 35 wt% Cr. More preferred are AISI 300 series steels such as AISI 301, 303, 304, 309, 310, 316, 316L, 317, 317L, 321, 347, CF8M, CF3M, 254SMO, A286 and AL6XN stainless steels. AISI 400 series stainless steels, especially Alloy 410, Alloy 416 and Alloy 440C are also particularly important.

Other classes of alloys that may be treated by the present invention include nickel-based, cobalt-based, and cobalt-based alloys which also contain sufficient chromium, typically about 10% or more, to form a protective layer of coherent chromium oxide upon exposure to air. And manganese based alloys. Examples of nickel-based alloys include Alloy 600, Alloy 625, Alloy 825, Alloy C-22, Alloy C-276, Alloy 20 Cb and Alloy 718 in some examples. Examples of such cobalt based alloys include MP35N and Biodur CMM. Examples of such manganese based alloys include AISI 201, AISI 203EZ and Biodur 108.

Another class of alloys in which the present invention may be practiced is a titanium-based alloy. As is well known in the metallurgical arts, the alloy forms a coherent titanium oxide protective coating that is impermeable to the passage of nitrogen and carbon atoms upon exposure to air. Specific examples of such titanium-based alloys include Grade 2, Grade 4 and Ti 6-4 (Grade 5).

Certain phases of the metal to be treated in accordance with the present invention may be applied to any phase structure metal such as but not limited to austenite, ferrite, martensite, duplex (e.g., austenite / ferrite) It is not important in the sense of being able.

Activation with anoxic nitrogen halide salt

According to the present invention, a workpiece having a complicated shape made of a self-passivating metal and having a bailed layer in at least one surface area thereof can be produced by heating the oxygen-free nitrogen halide salt for simultaneous and / (I.e., de-passivated) by contacting the workpiece with the workpiece. Surprisingly, it has been found that such vapors provide nitrogen atoms and selective carbon atoms for surface hardening, as well as having the effect of readily activating the surface of a self-passivating metal despite the presence of a considerable veiling layer. Also, quite surprisingly, the inventors have found that such a work-activated workpiece can be surface hardened in a much shorter time than was possible in the prior art. For example, if it takes between 24 and 48 hours to achieve a suitable case (surface and hardened layer) in a conventional process for activation and subsequent subsequent low temperature surface hardening, the activation and subsequent blanks for low temperature surface hardening The inventive process can achieve a corresponding case at a time as little as two hours.

Without wishing to be bound by theory, it is believed that the oxygen-free nitrogen halide salt decomposes before and / or as a result of contact with the workpiece surface to produce both halide and nitrogen ions. It is believed that halide ions are thought to effectively activate the workpiece surface while nitrogen ions diffuse into the workpiece surface to cure the surface through low temperature nitrification. When the anoxic nitrogen halide salt also contains carbon, carbon atoms are liberated when the anoxic nitrogen halide salt decomposes, and the carbon atoms are also diffused into the surface of the workpiece together with the nitrogen atoms. In this case, the surface of the workpiece is hardened through low temperature carbo-nitriding.

Therefore, when an anoxic nitrogen halide salt is used for the activation according to the present invention, it is not necessary to supply additional nitrogen-containing compounds for the nitrification process since the anoxic nitrogen halide salt used for activation will supply the nitrogen atoms necessary for nitridation In the sense, it will be understood that activation and nitrification occur simultaneously. Likewise, when a carbon-containing anoxic nitrogen halide salt is used in the activation according to the present invention, since the carbon-containing anoxic nitrogen halide salt will provide nitrogen and carbon atoms necessary for carbo-nitriding, both nitrogen and carbon Activation and carbo-nitriding are carried out at the same time in the sense that there is no need to supply additional compounds or compounds containing them.

On the other hand, if it is desired to promote nitriding, carburization, and carbo-nitriding that occur as a result of an oxygen-free nitrogen halide salt, additional nitrogen-containing compounds that can be decomposed to produce nitrogen atoms for nitriding, Compounds containing both carbon and nitrogen atoms that can be decomposed to produce both nitrogen and carbon atoms for carbo-nitriding, or any combination thereof, may be added to the system. In some embodiments of the present invention, such additional nitrogen containing and / or carbon containing compounds will be added after the activation of the workpiece is completed. In the context of this disclosure, such techniques are referred to as "subsequent" low temperature nitriding, carburizing and / or carbo-nitriding treatments. In another embodiment of the present invention, such additional nitrogen-containing and / or carbon-containing compounds may be added either prior to termination of the workpiece or at the same time as the start of activation. In the context of this disclosure, such techniques are referred to as "simultaneous" low temperature nitridation, carburization and / or carbo-nitridation treatments.

Another way of expressing the above is that in the conventional manner in which the workpiece forms a hardened surface or "case" on the workpiece surface, depending on whether the activation of the present invention is accomplished or not, Or a low temperature carbo-nitriding treatment. As will be appreciated by those skilled in the art, this can be achieved by forming a gaseous compound and a gaseous compound that can be decomposed to produce both nitrogen atoms for nitridation, carbon atoms for carburization or both nitrogen and carbon atoms for carbo-nitridation to form nitride precipitates or carbide precipitates Lt; RTI ID = 0.0 > under all conditions avoiding < / RTI > For the sake of convenience, such low-temperature light-curing processors are referred to in this disclosure as "low temperature gas curing" or "low temperature gas curing processes"

The anoxic nitrogen halide salt that can be used for activation and surface hardening in accordance with the present invention comprises (1) a halogenated anion that provides a solubility in anoxic nitrogen halide salt of at least 5 moles / liter to water at ambient temperature , (2) contains at least one nitrogen atom, (3) does not contain oxygen, and (4) is heated to a temperature of 350 ° C at atmospheric pressure. In this regard, it should be noted that the anoxic nitrogen and fluorine containing compounds, i.e. NF ₃ , which have been used as activating gases in the prior art, are not anoxic nitrogen halide salts in the context of this disclosure in that they are not ionic and therefore not saline something to do.

Specific examples of useful anoxic nitrogen halide salts for this purpose include ammonium chloride, ammonium fluoride, guanidinium chloride, guanidinium fluoride, pyridinium chloride and pyridinium fluoride. Oxygen-containing nitrogen halide salts, such as ammonium chlorate and ammonium perchlorate, should be avoided as oxygen atoms liberated during decomposition interfere with activation (de-passivation). In addition, chlorate and perchlorate should be avoided for additional reasons that they may explode when heated to high temperatures.

To achieve activation of the workpiece in accordance with the present invention, the workpiece is exposed (i.e., brought into contact with) the vapors produced when the oxygen-free nitrogen halide salt is vaporized by heating. This can be achieved at atmospheric pressure, above atmospheric pressure, or under a hard vacuum, i. E. A total pressure of less than 1 Torr (133 Pa), as well as a soft vacuum, i.e. 3.5 to 100 torr )), Including atmospheric pressure.

As noted above, the exact mechanism that occurs when such a contact occurs is not clear at this time writing this specification. However, what is clear is that the surface of the workpiece is effectively activated (i.e., de-passivated) for simultaneous and / or subsequent carburization, nitriding and carbo-nitriding if it comes into contact with such steam for a suitable length of time at the appropriate activation temperature.

In this regard, it will be appreciated that, in a low temperature surface hardening processor, unwanted nitrides and / or carbide precipitates are formed if the workpiece is exposed to too high a temperature. In addition, the maximum allowable surface hardening temperature for the workpiece without forming nitride and / or carbide precipitates will depend on the particular form of the low temperature surface hardening process to be performed (e.g., carburizing, nitriding or carbo-nitriding) (E.g., nickel-based vs. iron-based alloys), and the concentration of nitrogen and / or carbon atoms diffused into the surface of the workpiece. See, for example, US 6,547,888 assigned to the assignee of the present invention. It will also be appreciated that in performing a low temperature surface hardening process, care must be taken to avoid too high a surface hardening temperature to avoid the formation of nitride and / or carbide precipitates.

Likewise, care must also be taken to ensure that the temperature at which the workpiece is exposed during activation is not too high to cause undesired nitrides and / or carbide precipitates to form in carrying out the process of the present invention. Generally, this means that the maximum temperature at which the workpiece is exposed during activation should not exceed 500 占 폚, preferably 475 占 폚 or even 450 占 폚, depending on the particular alloy to be treated. Further, for example, when the nickel-based alloy is surface hardened, the maximum activation temperature may be as high as about 500 占 폚, since the alloys do not form nitride and / or carbide precipitates until a higher temperature is reached. On the other hand, when an iron-based alloy such as stainless steel is surface hardened, it is preferable that the maximum activation temperature is limited to about 450 캜, because the alloys tend to be sensitive to the formation of nitride and / or carbide precipitates at high temperatures.

With respect to the minimum activation temperature, there is no substantial lower limit, except that the temperatures of both the oxygen-free nitrogen halide salt and the workpiece itself must be sufficiently high to be in a vaporized state when the oxygen-free nitrogen halide salt contacts the workpiece surface to be activated .

With respect to the minimum activation time, in many cases, the process of the present invention will be performed such that the workpiece is continuously exposed to the vapor of the oxygen-free nitrogen halide salt during the entire low temperature thermal curing process. In these cases, there is no minimum activation time since activation continues until the low temperature thermal curing process is terminated.

However, if the contact of the workpiece with the oxygen-free nitrogen halide salt vapors is terminated prior to the end of the heat curing process, the contact should last long enough for the workpiece to be effectively activated before the contact between the workpiece and its vapor is terminated. The time can be easily determined by routine experimentation based on the case. Generally speaking, however, the contact should last at least about 10 minutes, more typically at least about 15 minutes, at least about 20 minutes, at least about 30 minutes, at least about 45 minutes, or at least about 1 hour.

The amount of anoxic nitrogen halide salt used to activate a particular workpiece depends on the application in which the halide salt is being used. If the salt is used solely for activation, then the amount used is only needed to a sufficient degree to achieve effective activation. On the other hand, if the salt is used for activation, of course, to supply some or all of the nitrogen atoms necessary for nitridation (or some or all of both the carbon and nitrogen atoms necessary for carbo-nitriding) It should be sufficient to meet all uses. The quantities were determined by the following treatments 13 and 14, which used anoxic nitrogen halide salts primarily for activation, as compared to Treatment Examples 1 to 3, which used anhydrous nitrogen halide salts for both activation and nitration It can be different.

Thermal Hardening

Once the workpiece of the process of the present invention is sufficiently activated, the workpiece may be thermoset by an established method of low temperature nitriding, carburizing and / or carbo-nitriding. In other words, the manner in which the workpiece is processed after activation in terms of chemical composition, time, temperature and pressure of the reaction gas exposed in the reactor for the curing reaction, as well as the reactor in which the workpiece is processed is all common. In some cases, as described above, the workpiece may continue to be exposed to the vapor of the oxygen-free nitrogen halide salt during some or all of the heat curing process. In other cases, if desired, the exposure may be terminated, for example, by stopping the flow of oxygen-free nitrogen halide salt vapors to the reactor before the thermal cure is completed. In either situation, thermal curing is accomplished in such a way that a case of desired depth (i.e., a cured surface layer) is created in the workpiece in a manner that avoids the formation of nitride and / or carbide precipitates or other self- do.

Thus, if the particular thermal curing process being performed is a nitridation process, the workpiece is exposed to a nitridation temperature that is sufficiently high to allow nitrogen atoms to diffuse into the surface of the workpiece, but lower than the temperature at which nitride precipitates are formed, Is nitrided. Thus, if the particular thermal curing process being performed is a carburizing process, the workpiece is exposed to a carburizing temperature that is high enough to allow carbon atoms to diffuse into the surface of the workpiece, but lower than the temperature at which the carbide precipitates are formed, Is carburized. And, if the particular thermal curing process being performed is a carbo-nitriding process, the workpiece is exposed to a carburizing disease temperature that is sufficiently high to allow nitrogen and carbon atoms to diffuse into the surface of the workpiece, but lower than the temperature at which nitride precipitates or carbide precipitates form, Thereby nitriding the workpiece without forming nitride precipitates or carbide precipitates.

In a particularly interesting technique, the activation and thermosetting are effected in accordance with the invention in a completely sealed reaction vessel for the entry or exit of any substance during the entire system, i.e. during the entire activation and thermosetting process. To ensure that activation and thermosetting are properly carried out, it is desirable to bring a sufficient amount of nitrogen halide salt vapor into contact with the surface of the workpiece that needs to be activated, particularly with its surface area having a significant bailed layer. As a nitrogen halogenated salt used in both activation and thermosetting according to the present invention is usually a particulate solid, an easy way to ensure that the contact is properly made is to coat the surface with a particulate solid or otherwise, And then sealing the reaction vessel before starting the heating of the workpiece and the oxygen-free nitrogen halide salt. Alternatively, the anoxic nitrogen halide salt may be dissolved or dispersed in a suitable liquid and coated on the workpiece in such a manner.

This technique is particularly convenient when large batches containing a large number of small workpieces, such as ferrules and fittings for conduits, are simultaneously heat cured in the same reaction vessel.

The technique described above using a closed system resembles in some respects the technique disclosed in US 8,414,710 to Minemura et al. In which case the self-passivating metal workpiece to be surface hardened is an amino resin such as melamine resin, urea resin, aniline resin or formalin resin And then heated to de-passivate and heat cure the workpiece at the same time. However, the heat curing process disclosed therein is a conventional high temperature and plasma assisted nitriding and carburizing process. In addition, nitrogen halide salts are neither disclosed nor proposed. The technique of the present invention is particularly advantageous in that the nitrogen halide salt is used not only for activating a passivated metal having a bailed layer but also for hardening the metal without forming carbide and / And the like. The amino resins of the Minemura et al. Patent are not believed to function for such purposes.

In this regard, it has been found that other nitrogen and carbon containing compounds, which are more similar to the oxygen free nitrogen halide salts of the present invention than the amino resins of the Minemura et al. Patent, are not effective in terms of achieving the objectives of the present invention. Thus, for example, guanidinium carbonates, cyanuric acids, imidazoles and calcium cyanamides are preferred because they are similar in many respects to the oxygen free nitrogen halide salts of the present invention, but have a workpiece made of AISI 316 stainless steel with a bale non- It was confirmed that it would not be successfully activated for simultaneous cold carburization.

The technique of the present invention in which activation and thermosetting are performed in a closed system as described above also resembles the technique disclosed in US Pat. No. 3,232,797 to Bessen in some respects, in which case the forced strips are treated with guanidinium compounds And then heated to decompose the guanidinium compound to nitridize the steel strip. However, thin forced strips that are nitrided in the patent are not self-passivating in the sense that they form a cohesive oxide protective coating with strong adhesion that is impermeable to the passage of nitrogen and carbon atoms. Thus, the technique disclosed in that patent is based on the fact that stainless steels and other self-passivating metals that are impermeable to the passage of nitrogen and carbon atoms can be removed by contact with the vapors of anoxic nitrogen halide salts as part of a low- Are not related to the present invention, which is imparted with transparency to the above-mentioned materials.

Selective N / C compound

As described above, WO 2011/009463 (US 8,845,823) of Christiansen et al. Discloses that by exposing stainless steels and other self-passivating metals to vapors generated by heating and decomposing "N / C compounds" I can teach you that you can do it. As further described in that document, a separate activation step with a halogen-containing gas is not required, since the vapor generated by the decomposition of the N / C compound has been found to also activate such metals. However, as noted above, the inventors have found that such a compound can not achieve activation in an effective manner if the surface of the workpiece to be carbo-nitrated comprises a bailed layer.

On the other hand, according to an optional feature of the present invention, the activation process of the present invention can be enhanced by including at least one of the N / C compounds in the reaction system during the activation process, Respectively. Additionally or alternatively, such N / C compounds may also be used to supply some or all of the additional nitrogen and carbon atoms needed for subsequent carbo-nitriding treatment. In this regard, it should be understood that the additional nitrogen and carbon atoms needed for the "subsequent" carbo-nitriding treatment refer to the carbon and nitrogen atoms consumed during the carbo-nitriding that occurs after the activation of the workpiece is essentially complete.

A suitable N / C compound that can be used for such optional features includes (a) contains both nitrogen and carbon atoms, (b) comprises at least one nitrogen carbon-carbon bond, (c) contains at least four carbon atoms And (d) are present in solid or liquid state at a temperature of 20 ° C and a pressure of 1 atmospheric pressure (0.1 MPa). Specific compounds useful for this include urea, acetamide and formamide, with urea being preferred.

The amount of selective N / C compound that can be used to practice such a feature of the present invention is determined by whether the compound is intended only for the promotion of activation or to provide the compound with nitrogen and carbon atoms for subsequent Depending on whether you want to use it. In addition, the amount of oxygen-free nitrogen halide salt included in the system is more than that required for activation, and if so, depends on what the amount of excess is.

In any event, in the former context, where the selective N / C compound is intended to only be used to enhance activation, the amount of selective N / C compound utilized will be in the range of 5 to 150% by weight, 125% by weight, and even from 50 to 100% by weight.

The amount of selective N / C compound that can be used in the latter situation also depends on whether additional source compounds are used to supply a portion of the carbon and / or nitrogen atoms needed for subsequent carburization, nitridation or carbo-nitridation treatments. In any event, the present inventors have found that in that situation the amount of N / C compound used in both the activation and subsequent carbo-nitriding treatments can range from 0.5 to 1,000 times, more typically from 1 to 100 times, from 1.5 to 1.5 times the amount of anoxic nitrogen halide salt used for activation To 50 times, 2 to 20 times, or 2.5 to 15 times (or related thereto). In this regard, the present inventors have found that particularly good results are achieved when the N / C compound is used in excess of the anoxic nitrogen halide salt in carrying out the optional features of the present invention, 2 to 20 times, more typically 2.5 to 15 times, and even 3 to 11 times the amount of oxygen-free nitrogen halide salt to be used.

Exposure of workpieces to atmospheric oxygen

In another embodiment of the present invention, the workpiece is exposed to atmospheric oxygen after the activation of the workpiece is substantially complete.

As mentioned earlier, the traditional way of activating stainless steels and other self-passivating metals for low-temperature carburization and / or carbo-nitriding is to contact the workpiece with a halogen-containing gas. In this regard, in some earlier studies in the art as described in US 5,556,483, US 5,593,510 and US 5,792,282, the halogen containing gas used for activation is only present with fluorine containing gases, especially HF, F ₂ and NF ₃ Limited. This is because, when other halogen-containing gases, especially chlorine-containing gases, are used, the workpiece is again passivated as soon as it is exposed to atmospheric air between activation and thermosetting. On the other hand, when a fluorine-containing gas is used for activation, re-passivation does not occur.

The fluorine-containing gas is highly reactive and highly corrosive and expensive, so it is desirable to avoid the use of the gas to avoid such problems. On the other hand, the use of a chlorine-containing gas for activation requires that the workpiece is not exposed between the activation and thermosetting in the atmosphere, which also requires activation and thermosetting to be performed in the same furnace . Thus, with regard to activating the self-passivating metal for thermosetting, there is an inherent trande-off between using a fluorine-based activator and a chlorine-based activator, i.e., the fluorine- And the expense, while the chlorinated activator is a realistic problem, suggesting activation and heat treatment in the same furnace.

According to a further feature of the present invention, the activated workpiece produced by the present invention is not easily re-passivated when exposed to atmospheric oxygen, even when the anoxic nitrogen halide salt used for activation is a chloride rather than a fluoride The conflict was broken because it was confirmed. Namely, the chlorine-based oxygen-free nitrogen halide salt has the same method as the fluorine-based anoxic nitrogen halide salt even if the exposure lasts for 24 hours or more in terms of producing an activated workpiece that is not readily passivated again upon exposure to atmospheric air In the present invention.

As a result, there is no longer a need to choose between activating the workpiece for low temperature curing and using a fluorinated activator on the one hand and activating and heat-treating the same furnace on the other hand. On the other hand, in the practice of the present invention, activation and heat treatment can be carried out in two completely different furnaces, without taking precautions to avoid exposure of the workpiece to atmospheric air, even with chlorinated activators, if desired. This technique, i.e., the use of separate activation furnaces and heat treatment furnaces, is inherently simple in terms of both furnace operation and capital cost, thus making the overall process less expensive to perform.

As described above, exposure to atmospheric oxygen in accordance with aspects of the present invention may occur at any time after the activation of the workpiece is substantially complete. What this means is that in terms of implementation, exposure to atmospheric oxygen must be delayed until the workpiece becomes sufficiently active to avoid substantial re-passivation. In other words, the exposure should not be so early as to cause a major negative impact on the process of the subsequent low temperature curing process due to the use of improperly activated workpieces. Except for these limitations, in the present invention, exposure of the workpiece to atmospheric air can occur at any time, including after the start of the subsequent low temperature thermal curing process.

Exposure to air in the atmosphere, however, will occur as a general consequence of moving the workpiece from the activation furnace between activation and low temperature thermal cure to a separate thermosetting furnace.

Processing Example

In order to explain the present invention more specifically, the following processing examples are provided.

Example 1:

The cut of a 1/2 inch (1.27 cm) diameter machined ferrule made of AISI 316 stainless steel is enclosed in a 12 mm diameter 210 mm length vacuum (1 to 2 Pa) glass ampoule with 10 g of guanidinium chloride Respectively. The ampoule was heated to 720 K (447 ° C) at a rate of 50 K / min in the furnace to volatilize the guanidinium chloride. After 2 hours at 720 K, the ampoule was removed from the furnace and rapidly cooled. Metallurgical analysis of the subsequent cross-section cutting ferrules revealed diffusion formation of the carbo-nitrided case at 37 탆 depth with near surface hardness of 1000 Vickers (29 g indentation load).

Examples 2 and 3:

Example 1 was repeated two times and three times. In the second case, it was confirmed that the case depth was 38 mu m and the surface hardness was 1300 Vickers. In the third case, it was confirmed that the case depth was 36 mu m and the hardness around the surface was 1200 Vickers. These examples demonstrate that the technique of the present invention is highly reproducible.

Example 4:

The workpiece (i.e., the cut-off portion of the ferrule as it was processed) was sealed with 0.01 g of NH ₄ Cl and 0.11 g of element, the glass ampoule was 220 mm long and the ampoule was heated to 450 ° C. for 120 minutes Except for this, Example 1 was repeated. This example was performed four separate times. All of the carbo-nitriding workpieces had a near surface hardness of about 1200 Vickers and a uniform case depth of 15, 18, 18 and 20 mu m, respectively.

Example 5:

Example 4 was repeated except that the workpiece was enclosed with 0.01 g of gallium anilide and 0.11 g of element. This example was also carried out four times separately. The obtained carbo-nitriding workpieces all had a hardness near the surface of about 1100 Vickers and a uniform case depth of 20 mu m, 21 mu m, 22 mu m and 18 mu m, respectively.

Example 6:

Example 4 was repeated except that the workpiece was enclosed with 0.01 g of pyridinium chloride and 0.11 g of element. This example was performed only once, yielding a carbo-nitrided workpiece having a near surface hardness of about 900 Vickers and a uniform case depth of 13 mu m.

Example 7:

The workpiece was sealed with 0.09 g of element, 0.03 g of a salt mixture containing 10% by weight of pyridinium chloride, 10% by weight of anhydrous anodic chloride, 80% by weight NH ₄ Cl and the workpiece was first treated with 250 Lt; RTI ID = 0.0 > 450 C < / RTI > for 120 minutes. The prepared carbo-nitriding workpiece had a near surface hardness of about 850 Vickers and a uniform case depth of 14 mu m.

Example 8:

Example 7 was repeated except that the workpiece consisted of 825 Incoloy alloys. The carbo-nitrided workpiece produced had a near surface hardness of about 600 Vickers and a uniform case depth of 12 mu m.

Example 9:

Example 8 was repeated except that the workpiece consisted of 625 Incoloy alloy. The prepared carbo-nitriding workpiece had a hardness near the surface of about 600 Vickers and a uniform case depth of 10 mu m.

Example 10:

A 1/4 inch machined state of the 625 Inconel alloy ferrule was enclosed in a 12 mm diameter 210 mm long vacuum glass ampule with 0.093 g of urea, 0.003 g of guanidinium chloride and 0.024 g of NH ₄ Cl Heated to 500 < 0 > C for 120 minutes. This experiment was performed twice. Both of the carbo-nitrided workpieces produced had near surface hardness of about 1100 Vickers while one of the workpieces had a uniform case depth of 14 mu m and the other had a uniform case depth of 11 mu m.

Example 11:

Example 10 was repeated, except that the workpiece consisted of a cut of the ferrule as machined in a 1/2 inch of the 825 Incoloy alloy. Both of the carburized nitrides produced had a near surface hardness of about 1250 Vickers while having a uniform case depth of 20 and 22 mu m, respectively.

Example 12:

The first workpiece, which includes cuts of ferrule in machined form, is made of three separate workpieces, AISI 316 stainless steel, 1/2 inch (1.27 cm) in diameter, machined as a 1/4 inch of 625 Inconel alloy And a third workpiece, including a cut of a ferrule of 1/2 inch machined state of an 825 Incoloy alloy, were placed together in a glass cylinder having a 12 mm diameter and 120 mm long open end. In addition, 0.63 g of guanidinium chloride, 5.0 g of NH ₄ Cl and 19.4 g of urea were placed in a glass tube with an open end. The tube was then heated at 470 [deg.] C for 120 minutes.

The carbo-nitrided product obtained from the AISI 316 stainless steel ferrule had a uniform case depth with a depth of 12 [mu] m and a hardness near surface of about 1000 Vickers. The carbo-nitrided product obtained from the ferrule of the machined state of the 625 Inconel alloy had a uniform case depth of 8 μm and a hardness near the surface of approximately 800 Vickers, while the carburization obtained from the ferrule of the machined state of the 825 Incoloy alloy The nitrated product had a uniform case depth of 11 mu m and a surface hardness of about 1200 Vickers.

This example shows that both the nitrogen chloride salt activation and the cold element-based carbo-nitriding process proceed simultaneously with the open air oven. In other words, the oxygen in the atmospheric air hardly interfered with the effective activation of the self-passivating metal for the low temperature carbo-nitriding treatment.

Example 13:

Two separate workpieces, each containing a cut of a machined ferrule of 1/2 inch (1.27 cm) diameter made of AISI 316 stainless steel, were encapsulated in the same 2 mm diameter 120 mm length ampoule, 0.13 g of NH ₄ Cl was also accommodated. The ampoule was evacuated to 1 to 2 Pa pressure, sealed and heated in an activation furnace at 350 DEG C for 60 minutes. The ampoule was then cooled, crushed open, and the two workpieces in it were carried in an open atmosphere with two separate carburizing furnaces located several miles away from each other.

After being exposed for about 24 hours in an open atmosphere, each workpiece was subjected to a low temperature carburizing treatment by contact with carburizing gas at 450 占 폚 for 16 hours. The carburizing gas used in the first carburizing furnace was composed of 27% of acetylene, 7% of H ₂ and 66% of N ₂ . On the other hand, the carburizing gas used in the second carburizing furnace was composed of 50% of acetylene and 50% of H ₂ .

The carburized workpiece made in the first carburizing furnace had a near surface hardness of about 1000 Vickers and a uniform case depth of 20 mu m while the carburized workpiece made in the second carburizing furnace had a surface roughness of about 750 Vickers Hardness and a uniform case depth of 20 mu m.

This example shows that although the oxygen-free nitrogen halide salt used to activate the workpiece is chlorine-based, the activated workpiece is essentially unaffected by exposure to atmospheric oxygen.

Example 14:

Example 13 was repeated except that an ampoule containing two workpieces was heated at 350 DEG C for 90 minutes. The carburized workpiece made in the first carburizing furnace had a near surface hardness of about 1000 Vickers and a uniform case depth of 20 μm while the carburized workpiece produced in the second carburizing furnace had a surface roughness of about 800 Vickers Hardness and a uniform case depth of 20 mu m.

While only a few embodiments of this alias have been described above, it will be appreciated that numerous modifications can be made without departing from the spirit and scope of the invention. All such modifications are intended to be included within the spirit and scope of the present invention, which is limited only by the following claims.

Claims (20)

A workpiece made of a self-passivating metal having at least one surface area that forms a Beilby layer as a result of a previous metal forming process is activated for subsequent cold carburization, carburizing or nitriding treatment As a process:
Exposing the workpiece to contact with vapors produced by heating the anoxic nitrogen halide salt to a temperature high enough to convert the oxygen-free nitrogen halide salt to steam
Wherein the workpiece is exposed to the steam for a time sufficient to activate the workpiece at an activation temperature that is lower than the temperature at which nitride and / or carbide precipitates are formed. The method of claim 1, wherein the anoxic nitrogen halide salt comprises: (1) a halogenated anion that provides a solubility of at least 5 moles / liter to water in the anoxic nitrogen halide salt at room temperature; (2) (3) oxygen-free, and (4) atmospheric pressure to 350 < 0 > C. 3. The process of claim 2, wherein the anoxic nitrogen halide salt comprises ammonium chloride, ammonium fluoride, guanidinium chloride, guanidinium fluoride, pyridinium chloride, pyridinium fluoride, or mixtures thereof. 4. The process of claim 3, wherein the anoxic nitrogen halide salt comprises ammonium chloride, guanidinium chloride, or mixtures thereof. 5. The process of claim 4, wherein the anoxic nitrogen halide salt comprises ammonium chloride. 5. The process of claim 4, wherein the anoxic nitrogen halide salt comprises guanidinium chloride. The process according to claim 1, wherein the workpiece is made of stainless steel. 8. The process according to claim 7, wherein the stainless steel is made of AISI 316 stainless steel. 7. The process according to any one of claims 1 to 6, wherein the workpiece and the oxygen-free nitrogen halide salt are heated together in a closed system. 2. The process of claim 1, wherein the workpiece is exposed to the steam under sufficient time and temperature conditions to allow nitrogen atoms to diffuse into the surface of the workpiece, thereby nitriding the workpiece without the formation of nitride precipitates. 11. The method of claim 10, wherein the oxygen free nitrogen halide salt comprises carbon and the workpiece is exposed to the vapor under conditions of sufficient time and temperature to allow both nitrogen and carbon atoms to diffuse into the surface of the workpiece, Or carburizing the workpiece without the formation of carbide precipitates. 12. The method of claim 10 or 11, wherein the workpiece is subjected to a low temperature gas hardening treatment selected from the group consisting of a low temperature carburizing treatment, a low temperature nitriding treatment, and a low temperature carburizing nitriding treatment, Wherein the cold gas hardening treatment is performed by contacting the workpiece with an additional gas other than the vapor and the additional gas is a compound capable of decomposing to produce a nitrogen atom for nitridation, A compound capable of decomposing to produce carbon atoms for carburization, and a compound capable of decomposing to produce both nitrogen and carbon atoms for carbo-nitriding. 13. The process of claim 12, wherein the workpiece contacts the additional gas only after the workpiece is activated. 13. The process of claim 12, further comprising exposing the workpiece to atmospheric oxygen after the workpiece is activated. 15. The process of claim 14, wherein the anoxic nitrogen halide salt is a chloride. 15. The method of claim 14, wherein activation of the workpiece is performed in an activation furnace, wherein low temperature gas curing is performed in a heat treatment furnace different from the activation furnace, and wherein the workpiece is transferred between the activation furnace and the heat treatment furnace Wherein the process is exposed to atmospheric oxygen. 17. The process of claim 16, wherein the anoxic nitrogen halide salt is a chloride. The process of claim 1, further comprising exposing the workpiece to atmospheric oxygen. 19. The process of claim 18, wherein the anoxic nitrogen halide salt is a chloride. 20. The process of claim 19, wherein the workpiece is exposed to atmospheric oxygen prior to a subsequent cold carburizing, nitriding or carbo-nitriding treatment.